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1. Measurement and modeling of aerosol Fe speciation Jingqiu Mao (GFDL/NOAA), Songmiao Fan (GFDL/NOAA), Ying Chen (Fudan U, China)
2. Why do we care about aerosol Fe? A dominant source of nutrient iron to open ocean, critical for plankton in surface waters: -Biological pump of CO2 -DMS production->sulfate->marine clouds->climate Ocean Fe is mainly supplied by dust (95%) Phytoplankton blooms in the South Atlantic Ocean. (MODIS) “Give me a half a tanker of iron and I'll give you the next ice age”- John Martin
3. Fe(II) is the bioavailable form of aerosol Fe Solubilization of dust Fe Fe(II) solubilities ~ 0.1% Crystal structure of hematite
4. Solubilization of dust Fe by atmospheric processing Soil has low Fe solubilities ~ 0.1% Solubilities of aerosol Fe in remote regions: up to 80% Fine aerosols (<2.5 µm) tend to yield larger iron solubilities than coarse aerosols (Siefert et al., 1999; Baker et al., 2006) Solubility Aerosol mass (Baker et al., 2006)
5. Solubilization of dust Fe Dust Fe(III)= Fe3+ + Fe(OH)2+ + Fe(OH)2+ + Fe(SO4)+ + … Fe(II)= Fe2+ + Fe(OH)++ Fe(SO4) + …
6. Aqueous Fe chemistry Fe(II) + HO2 Fe(III) + H2O2 Fe(II) + H2O2 → Fe(III) + OH + OH− Fe(II) + OH → Fe(III) + OH− Fe(II) Fe(III) ??? Fe(III) + H2O2/HO2 are too slow to be important
7. Current mechanisms for Fe(III)→Fe(II) (1) Enhanced photolysis of Fe(III) by cloud processing aerosol cloud ? Fe(OH)2+ + hv Cannot maintain the steady state of Fe(II)/Fe(III) after clouds evaporate. Cloud: pH~4 Aerosols: pH<3 (Zhuang et al., 1992, Nature)
8. Current mechanisms for Fe(III)→Fe(II) (2) Enhanced photolysis by organic acids Fe2+ + CO2 (Zuo and Hoigné, 1992) (photolysis rate ~ 10-2 s-1) Limitation: need continuous supply of oxalic acid in aerosols. We need something to sustain the steady state of Fe(II)/Fe(III)!
9. Current mechanisms cannot explain nighttime Fe(II) measurements!! Fe(II) + H2O2 → Fe(III) + OH + OH− Lifetime of Fe(II)< 1hr for 1ppb H2O2 aerosol Fe(II) measurements in marine boundary layer Fe(III)→ Fe(II) ??????? N-nighttime D-daytime O2- is the only electron donor we can think of at night, but Fe(III) + O2 − is too slow. However, Cu(II) + O2− is faster by a factor of 300 Cu(I) + Fe(III) is very fast (Zhu et al., 1997)
10. A new driver for aqueous Fe(II) production-HO2 uptake Cu(II) +HO2 → Cu(I) + O2 + H+ electron transfer reaction (very fast) Cu(I) + Fe(III) → Cu(II) + Fe(II) Fe(II) + HO2 Fe(III) + H2O2 Fe(II) + H2O2 → Fe(III) + OH + OH− Fe(II) + OH → Fe(III) + OH− Fe(II) is sustained by gas-phase HO2!!!! Cu-Fe redox coupling
11. Nighttime Fe(II) can be supplied by nighttime HO2 HO2 measurement over remote ocean HO2 >0 (Kanayaet al., 2000) Unique role of HO2 in heterogeneous chemistry: (1) its lifetime (~ 1-10 min), long enough for het chem (OH lifetime only ~1 s). (2) high polarity in its molecular structure. (very soluble compared to OH/CH3O2/NO/NO2). (3) very reactive in aqueous phase (a major reason for DNA damage and cancer, superoxide).
12. Gas phase HO2 uptake by particles aerosol HSO4- NH4+ NH4+ HO2 HO2(aq) SO42- ① ② ③ ④ NH4+ Aqueous reactions HSO4- SO42- HSO4- NH4+ γ(HO2) defined as the fraction of HO2 collisions with aerosol surfaces resulting in reaction. HSO4- SO42- NH4+ SO42- ① ③ ④ ②
13. Modeling framework for HO2 aerosol uptake aerosol Aqueous chemistry include Cu, Fe, Cu-Fe coupling, odd hydrogen and photolysis. Rout HO2 [HO2]surf Rin [HO2]bulk [HO2]surf Uptake rate [HO2]surf is higher than [HO2]bulk because of its short lifetime. Volatilization rate The diffusion equation with chemical loss (kI[HO2]) and production (PHO2) Chemical loss rate provides a relationship between [HO2]surf and [HO2]bulk.
14. Chemical budget for NH4HSO4 aerosols at RH=85%, T=298 K Cu/Fe = 0.05, HO2(g) = 10 pptv, H2O2(g) = 1 ppb Fe(III) reduction is dominated by Fe(III) + Cu(I), instead of photoreduction(implications for dust iron to ocean…) This process is entirely driven by HO2 (γ(HO2) = 0.7) OH budget is controlled by TMI.
15. Fe(II)/Fe ratio modulated by gas-phase HO2concentrations Field measurements of Fe(II)/Fe_total in MBL Fe(II)/Fe_total Higher Cu/Fe ratio leads to higher Fe(II)/Fe_total
16. Future measurements to test such mechanism
17. Transitional metal is abundant in crustal and combustion aerosols Transitional metals have two or more oxidation states: - -e - - Fe(II) Fe(III) - - + + + e - - - - - - Redox coupling driven by gas-phase HO2 uptake -e + - Cu(I) Cu(II) - - - - - - + + + e - - - - - - + - - - reduction(+e) + oxidation(-e) = redox
18. What else in dust aerosols? Measurements from dust aerosols There are tens of transitional metals in dust aerosols. We don’t know chemical kinetics for most of them. (Sun et al., 2005)
19. Ocean Fe supply is driven by atmospheric HO2 aerosol uptake! HO2
20. Cu and Fe are ubiquitous in crustal and combustion aerosols Cu/Fe ratio is between 0.01-0.1 IMPROVE Cu is fully dissolved in aerosols. Fe solubility is 80% in combustion aerosols, but much less in dust.
21. Ionic strength correction for aerosol aqueous chemistry Ideal solution (cloud droplets) Non-ideal solution (aqueous aerosol) Ai is activity coefficient for any species and also a function of ionic strength. - - - - - + + + - - - - - - - + - - - - - - Non-ideal behavior due to the electrostatic interactions between the ions. + - + + - - - - - - - + - - - Use Aerosol Inorganic Model (AIM) to calculate the ionic strength and activity coefficients for major ions (i.e. NH4+, H+, HSO4-, SO42-). Calculate activity coefficients for trace metal ions and neutral species based on specific ion interaction theory. Account for salting-out effect on Henry’s law constant.
22. Dependence on aerosol pH and Cu concentrations Cu/Fe=0.1 Cu/Fe=0.01 typical rural site γ(HO2) in the range 0.4-1 at T = 298 K, should be close to 1 at lower T, due to higher solubility. H2O2 yield is more likely to be negative than positive. HO2 uptake is limited by aqueous diffusion until Cu = 5 x 10-4 M.
23. Organic aerosols (insoluble organic) Organic-electrolyte mixtures tend to have liquid-liquid phase separation state. (Zuend et al., ACP, 2012) Water soluble organic aerosols Fe(III)C2O4 and Fe(II)C2O4 complexes are very unstable. Cu complexes can also be a significant sink for aqueous HO2 (Voelker et al., EST, 2000) (Furukawa et al., ACP, 2010)